Method and bath for electropolishing



Patented Sept. 5, 1950 UNITED STATES P' METHOD AND BATH FORELECTROPOLISHING Haroldd. W'iesner, Elkhar Gonn, Lt'dz, Elkhart,

diana t, Ind., assignor to G. G..

Ind., a corporation of In- No Drawing. Application January 19,- 1946,Serial'No. 642,435

16 Claims.

This invention relates to a methodof andbath for electropolishing, andmore particularly to the electropolishing of copperand copper alloys.

One of the objects of the invention isto. provide a method of and bathfor electropolishing in which all of the exposed surfaces of an articleto be polished will be'given a uniform bright finish free fromdiscoloration.

Another object is to provide a method. of and bath for electropolishingcopper alloys containing lead which will produce a high. luster freefrom smut.

Still another object is to provide a method of and bath forelectropolishing in which the bath is mildly alkaline.

A further object is to provide an electropolishing bath which isnon-corrosive so that relatively inexpensive equipment can be used,which is harmless to operators. either from contact or from fumesand'which requiresno venting equipment.

According to the present invention, articles to be polished, which maybe. any shape-or configuration of metal articles formedofcopper orcopper containing alloys, areconnected'as an anode in the. polishingbathand have acurrent passed through them to a cathode. in the bath which ispreferably copper or plated with copper. With the bath of the presentinvention current densities substantially inexcess. of. 100" amperes persquare foot can be used to produce a uniform high luster over the entiresurface of the articles, including recessedportions. The time requiredto produce a brilliantsurface is dependent upon the finish of thearticle prior to the electropolishing operation. It is apparent that forthis reason no specific time for accomplishing the electropolishing to ahigh luster can be given,.but in general the time willyary fromfi to 30minutes, andon anaverage will beabout 15to2'0 minutes.

The bath of this invention consists essentially of pyrophosphates of thealkali metals andeither borates or carbonates mixed therewith in anaqueous solution. The most satisfactory pyrophosphates are sodiumpyrophosphate, which is preferably used in the anhydrous ordeca-hydrate, and potassium pyrophosphate, which is preferably thetrihydrate. These salts function in the solution to form. solublecompounds with the copper, zinc, and lead andother metals commonly foundin the brass alloys during electropolishing. In the operation of thebath the copper plates out. as a sludge at the cathode so that the netloss of pyrophosphate is that which combines with the-zinc, lead andother metals. The

bath can therefore be used for relatively long periods of time with theaddition of only very small amounts, if. any, of pyrophosphates tomaintain its strength. I havefound that a satisfactory-concentration ison the order of 16 ounces brightening in therecesses of the work andother low current density areas, and etching mayoccur. Also,discolorations may occur in various areas of the work pieces. Among thespecific com.- poundswhich I- have'found to be most satisfactory areboric acid, sodiumtetraborate, potassium tetraborate, ammoniumcarbonate,sodium bicarbonate; and potassium bicarbonate, which are: preferablyused in concentrations offrom 2 to' 12 ounces per gallon. Ifammoniumrcarbonate is used, it is. necessary to: keep the temperature ofthe bath'below about 135 F., as above this temperature it decomposesrapidly. Ammonium carbonate and sodium and potassium'bi carbonatealsoreact with acid, which may be added to the bath to lower its: pHtogive off some carbon dioxide. Boric acid and sodium or potas siumtetraborate are preferred since they are thermally stable, do notdecomposewhen. acid is added, and function to buffer the solution.

In addition to these basic constituents of the bath there may be addedeither an acid --or base in quantities sufficient to bring the pI-I-vofthe bath in the range from 7.5 to 9.5. For this purpose sodium,potassium or ammonium hydroxide may be convenientl used to raise the pH,while sulfuric or meta phosphoric, and in some cases ortho phosphoric,acid may be used to lower the pH. If the pH of the bath is below about7.5, the satisfactory current density range is substantially narrowed,while if the pI-f ist'oo high, a highly lustrous surface cannot beobtained. More specifically, if the pH drops below the aforementionedlower limit and electropolishing' is attempted on a complicated piece ofwork, it has been found. that brown to redbr'own discolorations will beapparent on the high current density areas.- As the pH of the solutionis raised (with sodium hydroxide, for example) the current density rangeis broadened, and at a pH of 9.0 (for example) extremely high currentdensities may. be used without burning.

In baths containing ammonium carbonate, sodium hydroxide may be usedsatisfactorily to raise the pH; but in those baths containing boric acidor sodium tetraborate, ammonium hydroxide or an ammonium compound wasfound to give'better results. To lower the pH sulphuric, meta-phosphoricor ortho-phosphoric acids may be used satisfactorily but certain otheracids sodium, di potassium and di ammonium phosphates, when put intosolution, have pH values nearer to those in the operating range of thebath and for this reason are preferred. These salts, when added to thebath in concentrations of from 12 to ounces per gallon extend thecurrent range and greatly increase brightness.

It has further been found that the addition of oxalic acid or one of itssalts such as sodium oxalate, ammonium oxalate or potassium oxalate inconcentrations of about 4 to 6 ounces per gallon enables the temperatureof the bath to be increased up to as much as 200 F. with good results.Without the addition of these materials, it has been found that thesurface tends to become irregular at temperatures substantially inexcess of 150 F.

In general, I have found that agitation of the work or of the solutionduring electropolishing is desirable. As a result of using agitation,higher current densities are permitted, with less chance of burning theprotruding sections of a complicated piece of work. The commonlyemployed methods of agitation used in the electroplating industry, suchas oscillation of the work rod, air agitation, or solution agitation bmeans of pumps or paddles are satisfactory. The use of work rodoscillation is perhaps to be preferred.

As examples of several baths which I have found to be satisfactory, thefollowing may be iven:

Example #1 Sodium pyrophosphate (anhydrous) 16 oz./gal. Boric acid 5oz./gal. Ammonium hydroxide (28 B.) 2 ll. oz./gal. pH 8.5-9.0Temperature 140-160 F. Current density Above 100 amps/sq. ft.

Example #2 Potassium pyrophosphate 16 oz./gal. Boric acid 4-6 oz./gal.pH 8.5-9.0 Temperature 140-160" F. Current density Above 100 amps/sq.ft.

Example #3 Potassium pyrophosphate 16 oz./gal. Di potassium hydrogenphosphate 5-12 oz./gal. Boric acid 4-6 oz./gal. pH 8.0-8.5 Temperature140-160 F. Current density Above 100 amps/sq. ft.

Example #4 Sodium pyrophosphate (anhydrous) l6 oz./gal. Disodiumhydrogen phosphate 12-20 oz./gal. Boric acid 5 oz./gal. pH 8.0-8.5Temperature 130-150 F.

Current density Above 150 amps/sqf alloys of copper 4 Example #5 Sodiumpyrophosphate 16 oz./gal. Boric acid 5 oz./gal. Sodium oxalate or oxalicacid 4-6 oz./gal. pH 8.0-8.5 Temperature 180-200 F. Current densityAbove 150 amps/sq. ft.

While the invention has been particularly described in connection with.several specific bath compositions, it will be understood that variouscombinations and concentrations of the constituents mentioned, as wellas various operating conditions might be employed within the scope ofthe invention. The examples given are to be taken as illustrative only,reference being had to the appended claims for a determination of thescope of the invention.

What is claimed is:

l. The method of electropolishing copper and alloys of copper comprisingconnecting the part to be polished as the anode in a bath consistingessentially of water as the solvent, an alkali metal salt present in anamount ranging from 3 ounces per gallon of solution to a saturatedsolution of the salt, and from 2-12 ounces per gallon of solution of awater soluble salt selected from the group consisting of boric acid,water soluble salts of boric acid and water soluble salts of carbonicacid.

2. The method of electropolishing copper and alloys of copper comprisingconnecting the part to be polished as the anode in a bath consistingessentially of water as the solvent, an alkali metal salt present in anamount ranging from 8 ounces per gallon of solution to a saturatedsolution of the salt, and from 2-12 ounces per gallon of solution of awater soluble salt selected from the group consisting of boric acid,water soluble salts of boric acid and water soluble salts of carbonicacid, and maintaining the bath between a pH of 7.5 to 9.5.

3. The method of electropolishing copper and comprising connecting thepart to be polished as the anode in a bath consisting essentially ofwater as the solvent, an alkali metal salt present in an amount rangingfrom 8 ounces per gallon of solution to a saturated solution of thesalt, and from 2-12 ounces per gallon of solution of a water solublesalt selected from the group consisting of boric acid, water solublesalts of boric acid and water soluble salts of carbonic acid, andoperating the bath at a temperature of between -212 F. and at a currentdensity of in excess of 100 amperes per square foot.

4. The method of electropolishing copper and alloys of copper comprisingconnecting the part to be polished as the anode in a bath consistingessentially of water as the solvent, an alkali metal salt present in anamount ranging from 8 ounces per gallon of solution to a saturatedsolution of the salt, and from 2-12 ounces per gallon of solution of awater soluble salt selected from the group consisting of boric acid,water soluble salts of boric acid and water soluble salts of carbonicacid, and operating the bath ata pH of 7.5-9.5, at a temperature between100 F.2-12 F., and at a current density in excess of 100 amperes persquare foot.

5. A composition for providing an electropolishing bath when dissolvedin water, the ingredients being present in the following proportions pereach gallon of solution to be made, said com- 2 52 1, roe

position consisting essentially of an: alkali metal pyrophosphatepresent in an amount ranging from 8"ounces tothe Saturation pointpergallon of solution, 2-12*'ounces of a water soluble compoundselectedifrom the group consisting of borie acid, water soluble saltsofbori'c acid and water soluble salts of carbonic acid; and from 12-20Ounces of a salt of" ortho phosphoric acid in which the metalcomponentis-sel'ected from the group consisting ofa-mmonia, sodium: andpotas slum;

6. An electropolishing bath consisting essentially of water as thesolvent, an alkali metal pyrophosphate' present in an amount rangingfrom 8 ounces per gallon of solution to an amount saturatingthe bath,from 2-12 ounces per gallon of solution of a water soluble compoundselected from the group consisting of boric acid, water soluble saltsofboric acid, and water soluble salts ofcarbonic acid' and from 132 -20ounces of asalt of orthophosphoriaacid' in which the metallic componentis selected from the group consisting of sodium, potassium and ammonium.

'7. Acomposition for providing an electropolishing bath when dissolvedin water; the ingredients of which are present in the followingproportions for each gallon of solution, said composition consistingessentially of an alkali metal pyrophosphate present in an amountranging from 8 ounces to an amount to maintain saturated solution, 2-12ounces of a water soluble compound selected from the group consisting ofboric acid and water soluble salts of carbonic acid, a neutralizingagent in an amount to adjust the pH of the solution to between 7.5 and9.5, and from 12-20 ounces of a salt of ortho phosphoric acid in whichthe metallic component is selected from the group consisting ofammonium, sodium and potassium.

8. An electropolishing bath consisting essential- 1y of water as thesolvent, an alkali metal pyrophosphate present in an amount ranging from8 ounces per gallon of solution to an amount saturating the bath, from2-12 ounces per gallon of solution of a water soluble compound selectedfrom the group consisting of boric acid, water soluble salts of boricacid and water soluble salts of carbonic acid, a neutralizing agentpresent in an amount to adjust the pH of the bath between 7.5-9.5, andfrom 12-20 ounces of a salt of ortho phosphoric acid in which themetallic component is selected from the group consisting of ammonium,sodium, and potassium.

9. An electropolishing bath consisting essentially of water as thediluent, an alkali metal pyrophosphate present in an amount ranging from8 ounces per gallon of solution to a saturated solution of the salt, andfrom 2-12 ounces per gallon of solution of a water soluble compoundselected from the group consisting of boric acid, salts of boric acid,salts of carbonic acid, and from 4-6 ounces per gallon of a watersoluble material selected from the group consisting of oxalic acid andwater soluble salts of oxalic acid.

10. A composition for providing an electropolishing bath when dissolvedin water, the ingredients being present in the following proportions pereach gallon of solution to be made, said composition consistingessentially of an alkali metal pyrophosphate present in an amountranging from 8 ounces to an amount to maintain a saturated solution ofthe salt in water, and from 2-12 ounces of a water soluble compoundselected from the grou consisting of boric acid, water 6 soluble saltsof boric acid, water soluble" salts of carbonic acid, and from 4-6ouncesper gallon 01" a water soluble materialselected fromthe groupconsisting of oxalic acid and water soluble salts of oxalic'acid.

1 1 A composition for providing an electropolishing bath when dissolvedin water; the ingredients being present in thefollowing proportionspereach gallon of solution tobe made, said composition consistingessentially of an alkali metal pyrophosphate present in an amountranging from 8 ounces to the" saturation point per gallonof solution,242 ounces of a water soluble compound selected from thegroupconsistingof boric acid, water soluble" salts of boric acid and water solublesalts of carbonic acid; a neutralizing: agent in an amount to adjustthepI-I to between" 715-95, and from 4-6 ounces per gallon of a watersoluble material selected from the group consisting of oxalic acid andwater soluble salts of oxalic acid;

'2. A composition for providing anelectrospolishing bath when dissolvedin water, the ingredi'ents being present inthe following proper":t'i'onsper each gallon ofsol'ution to-be made; said compositionconsisting essentially of analkali metal pyrophosphate present in anamount ranging from 8 ounces to the saturation point per gallon ofsolution, 2-12 ounces of a water soluble compound selected from thegroup consisting of boric acid, water soluble salts of boric acid andwater soluble salts of carbonic acid, from 12 to 20 ounces of a salt ofortho phosphoric acid in which the metal component is selected from thegroup consisting of ammonia, sodium, and potassium, and from 1-6 ouncesper gallon of a water soluble material selected from the groupconsisting of oxalic acid and water soluble salts of oxalic acid.

13. An electropolishing bath consisting essentially of water as thesolvent, an alkali metal pyrophosphate present in an amount ranging from8 ounces per gallon of solution to an amount saturating the bath, from2-12 ounces per gallon of solution of a water soluble compound selectedfrom the group consisting of boric acid, water soluble salts of boricacid, and water soluble salts of carbonic acid, from 12-20 ounces of asalt of ortho phosphoric acid in which the metallic component isselected from the group consisting of sodium, potassium and ammonium,and from 4-6 ounces per gallon of a water soluble material selected fromthe group consisting of oxalic acid and water soluble salts of oxalicacid.

14. A composition for providing an electropolishing bath when dissolvedin water, the ingredients of which are present in the followingproportions for each gallon of solution, said composition consistingessentially of an alkali metal pyrophosphate present in an amountranging from 8 ounces to an amount to maintain saturated solution, 2-12ounces of a water soluble compound selected from the group consisting ofboric acid and water soluble salts of carbonic acid, a neutralizingagent in an amount to adjust the pH of the solution to between 7.5 and9.5, from 12-20 ounces of a salt of ortho phosphoric acid in which themetallic component is selected from the group consisting of ammonium,sodium and potassium, and from 4-6 ounces per gallon of a water solublematerial selected from the group consisting of oxalic acid and watersoluble salts of oxalic acid.

15. An electropolishing bath consisting essentially of water as thesolvent, an alkali metal pyrophosphate present in an amount ranging from8 ounces per gallon of solution to an amount saturating the bath, from2-12 ounces per gallon of solution of a water soluble compound selectedfrom the group consisting of boric acid, water soluble salts of boricacid and water soluble salts of carbonic acid, a neutralizing agentpresent in an amount to adjust the pH of the bath between 7.5-9.5, from12-20 ounces of a salt of ortho phosphoric acid in which the metalliccomponent is selected from the group consisting of ammonium, sodium, andpotassium, and from 4-6 ounces per gallon of a water soluble materialselected from the group consisting of oxalic acid and water solublesalts of oxalic acid.

16. The method of electropolishing copper and alloys of coppercomprising connecting the part to be polished as the anode in a bathconsisting essentially of water as the solvent, an alkali metalpyrophosphate present in an amount ranging from 8 ounces per gallon ofsolution to an amount saturating the bath, from 2-12 ounces per gallonof a water soluble salt selected from the group consisting of boricacid, Water soluble salts of boric acid and water soluble salts ofcarbonic acid, 12-20 ounces per gallon of solution of a salt of orthophosphoric acid in which the metallic component is selected from thegroup consisting of ammonium, sodium and potassium, from 4-6 ounces pergallon of solution of a water soluble material selected from the groupconsisting of oxalic acid and water soluble salts of oxalic acid,maintaining the bath at a pH between 7.5 and 9.5 and at a temperaturebetween 100-212 F. and operating the bath at a current density in excessof 100 amperes per square foot.

HAROLD J. WIE'SNER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. THE METHOD OF ELECTROPOLISHING COPPER AND ALLOYS OF COPPER COMPRISINGCONNECTING THE PART TO BE POLISHED AS THE ANODE IN A BATH CONSISTINGESSENTIALLY OF WATER AS THE SOLVENT, AN ALKALI METAL SALT PRESENT IN ANAMOUNT RANGING FROM 8 OUNCES PER GALLON OF SOLUTION TO A SATURATEDSOLUTION OF THE SALT, AND FROM 2-12 OUNCES PER GALLON OF SOLUTION OF AWATER SOLUBLE SALT SELECTED FROM THE GROUP CONSISTING OF BORIC ACID,WATER SOLUBLE SALTS OF BORIC ACID AND WATER SOLUBLE SALTS OF CARBONICACID.